A new study by Imperial College London (ICL) found that the risk of reinfection with the Omicron coronavirus variant is more than five times higher and it has shown no sign of being milder than Delta.
The results of the study were based on UK Health Security Agency and National Health Service data on people who tested positive for COVID-19 in a PCR test in England.
The new findings may add urgency to countries’ efforts to accelerate vaccination booster campaigns in a bid to ward off the burden on hospitals and health systems.
“We find no evidence (for both risks of hospitalization attendance and symptom status) of Omicron having different severity from Delta. Controlling for vaccine status, age, sex, ethnicity, asymptomatic status, region and specimen date, Omicron was associated with a 5.4-fold higher risk of reinfection compared with Delta,” the study said, although it added that data on hospitalizations remains very limited.
The protection afforded by past infection against reinfection with Omicron may be as low as 19 percent, ICL said in a statement, noting that the study has not yet been peer-reviewed.
The researchers found a significantly increased risk of developing a symptomatic Omicron case compared to Delta for those who were two or more weeks past their second vaccine dose and two or more weeks past their booster dose. The study involved AstraZeneca and Pfizer vaccines.
Depending on the estimates used for vaccine effectiveness against symptomatic infection from the Delta variant, this translates into vaccine effectiveness of between 0 percent and 20 percent after two doses, and between 55 percent and 80 percent after a booster dose.
“This study provides further evidence of the very substantial extent to which Omicron can evade prior immunity given by both infection or vaccination. This level of immune evasion means that Omicron poses a major, imminent threat to public health,” a study lead Professor Neil Ferguson said in ICL’s statement.
Dr. Clive Dix, former Chair of the UK Vaccine Taskforce, remarked that “The conclusions made are based on making assumptions about Omicron where we still don’t have sufficient data. For example, we have no data on the cellular immune response which is now probably driving the effectiveness of vaccines. This is a crucial missing assumption in the modeling.”
